This feels obvious, but I'm asking anyway since I can't find a clear confirmation in the documentation:
The semantics of the Flink Table API upsert kafka connector available in Flink 1.12 match pretty well the semantics of a Kafka compacted topics: interpreting the stream as a changelog and using NULL values as tombstone to mark deletions.
So my assumption is that it is ok to use it to consume from and produce to a compacted topic, and it's probably made precisely for that, although it should work fine as well with a non-compacted topic assuming its content is indeed a changelog. But I'm surprised not to find any reference to compacted topic in that part of the documentation.
Could somebody please confirm or infirm this assumption?
Yes, it was made for use with compacted topics. According to FLIP-149:
Generally speaking, the underlying topic of the upsert-kafka source must be compacted. Besides, the underlying topic must have all the data with the same key in the same partition, otherwise, the result will be wrong.